[WEB-2532] fix: custom theme mutation logic

[aaryan610]

Problem:

MutationObserver logic to update the custom theme is too expensive and unnecessary.

Solution:

Update the applyTheme function to select the themeElement with the attribute [data-theme] instead of [data-theme="custom"] to make sure themeElement always exists.

Plane issue: WEB-2532

Summary by CodeRabbit

• New Features

  • Enhanced theme application logic for a more streamlined user experience.

• Bug Fixes

  • Eliminated unnecessary dependencies on DOM elements, improving performance and reliability of theme settings.